Bits for drilling stone which are in current use have deep recesses with a circular profile having a steep curvature similar to the profile of the recesses in bits used for drilling metal. In order to ensure that these bits have adequate strength, the guide lips or rims therefor are necessarily relatively wide, their surface representing approximately 50% of the surface of the cylindrical casing of the bit. If the strength and guidance of the bit is adequate, then the frictional surface tends to be considerable, which causes rapid and excessive heating. Furthermore, the waste material, which occurs above all in the form of a powder with grains of varying size, tends to remain trapped in the recesses which are relatively narrow and deep, causing clogging, increasing the heating of the bit and the rotary torque and consequently the heating of the machine driving the bit, which consequently limits the operating speed, i.e. the speed of stone penetration. Fracture of the bit may also occur because it is subject to excessive torsional stress.
It has been proposed to improve the efficiency of these bits by reducing the width of the lips and by using recesses of quasi-rectangular trapezoidal profile, for example see West German Pat. No. 23 58 447. In order to ensure the mechanical strength of the bit, the width of the lips is still relatively great and the depth of the recesses relatively slight. Furthermore, dust tends to remain in the slightly rounded angles at the bottom of the recesses. However, these bits make it possible to increase the operating speed, i.e. the penetration speed, by approximately 30%.
Special profiles have been studied in order to improve the efficiency of bits having a diameter greater than 12 mm. However, these profiles cannot be used on bits of small diameters, since the strength of the body of the bit would be much too low.